Gas turbine engines are subjected to high temperatures during operation and consequently include components made from insulative materials. A porous oxide/oxide composite is one example of a high temperature insulative structural material used for a gas turbine engine component such as a combustion liner. One type of oxide/oxide composite that can be readily used in a combustion liner consists of alumina fibers embedded in an alumina matrix. Since this composite may weaken or degrade at temperatures in excess of 2200° F., one or more layers of a high temperature porous insulation material, typically a more or less friable oxide-ceramic based material such as alumina, are commonly used to provide a thermal barrier layer on top of the structural ceramic composite.
Since the oxide/oxide ceramic composite and the insulating material are porous, both can be permeated by gases such as water vapor. Gas turbine engine combustors typically operate in a high-pressure environment where water vapor typically makes up about 10% of the gas flowing through the combustor. This water vapor results from both vapor entrained with the combustion air and from vapor produced as a product of the combustion reaction process. A gas-permeable composite can react with the pressurized water vapor to produce volatile hydroxides, such as Al(OH)3 and Si(OH)4, depending upon the base oxide in the composite system. Volatile hydroxides can cause surface recession and particle-particle bond weakening within the base composite. An uninhibited water vapor attack can eventually cause a combustion liner or other component made of an oxide/oxide composite to lose some of its thickness and strength.
Hence, there is a need for environmental barrier coating materials that protect a ceramic or other composite from both high temperature, pressurized water vapor, and other chemical attacks. There is a further need for a ceramic composite system that include multiple layers of high temperature insulation or coating materials that inhibit water vapor permeation of the porous composite substrate and are suitable for high pressure and high temperature environments.